Voltage-controlled oscillator circuits are well known in the art, and may be generally described as circuits which generate an output signal, the frequency of which is determined by connecting a tank or resonant circuit. VCO circuits are important circuits in today's electronics as they form the crucial building blocks in larger circuits such as frequency synthesizers, modulators, demodulators, and clock recovery circuits typically employed in numerous telecommunication products.
In many instances, the performance of these telecommunications devices is limited by the VCO's phase noise, which can be generally described as the random variation in the phase or frequency of the output signal. In other words, the phase noise places a limit on how precisely the output frequency of the VCO can be generated. Phase noise in the output signal from the VCO can also result in cross-talk and increase bit error rates in digital systems. This loss in accuracy translates into errors in the transmission and reception of information.
FIG. 1 illustrates a prior art VCO circuit for reducing phase noise. As shown in FIG. 1, an inductor-capacitor (LC) tank circuit is employed to reduce the phase noise of the VCO. The LC tank includes inductors L1 and L2, variable capacitors C1 and C2, and a pair of cross-coupled transistors M1 and M2. The transistors M1 and M2 are biased by a bias current IB and a capacitor CS. The differential output signals are present at nodes V1 and V2.
The VCO circuit shown in FIG. 1 has several problems. First, the quality factor (Q) of the on-chip passive inductors L1 and L2 is limited. High Q inductors are necessary for reducing the phase noise of the VCO circuit of FIG. 1. One method for obtaining high Q inductors is to use gold-plated bond-wire to achieve high Q for the on-chip inductors L1 and L2. However, this solution is expensive. Another problem of the VCO circuit of FIG. 1 is that the voltage swing between gate and source of the transistors M1 and M2 is limited to the voltage swing of the output signals at V1 and V2, which is limited to the supply of the circuit. Because of this voltage swing limitation, the VCO circuit of FIG. 1 is more susceptible to phase noise than a VCO circuit that has a large voltage differential between the gate and the drain terminal. Hence, there is a need for a VCO circuit that exhibits improved phase noise performance.